The present invention relates generally to luminescent ink compositions, and more particularly, to compositions and methods for printing thin films of organic luminescent material useful in fabricating organic luminescent devices.
Printing is a simple and efficient way to create characters, graphics and various patterning on a substrate. Various printing technologies have been developed, such as stamping, screen printing, and ink-jet printing. Ink-jet printing is a non-contact dot-matrix printing technology in which droplets of ink are jetted from a small aperture directly to a specified position on a media. Solidification of the liquid by evaporation of solvent produce patterned thin films with an image. With the advancement of digital technology and the ease of ink control by computer, ink-jet printing process has received great attention in areas where images and information are created and modified digitally. Text and graphics printing are the two most applicable areas for ink-jet printing. The ink-jet printing process also has wide applications in other areas, such as textile printing. A variety of inks designed for these applications are known.
There are, however, only a few examples on the use of ink-jet printing in optoelectronic device applications, particularly in rapidly developing organic optoelectronic device field, such as photovoltaic cells, thin film transistors, and organic light-emitting diodes. Traditionally, these devices have been fabricated by sublimation (for low molecular compounds), spin-coating and dip-coating (for polymers). The use of screen printing technology for the fabrication of organic thin-film transistors was shown by Bao in 1996 using a pure poly(3-hexyl-thiophene) as the active polymer layer [Z. Bao, et al, Chem. Mater., 1997, 9, 1299].
With an aim to achieve lateral pattern control of organic luminescent films, ink-jet printing has been recently used to directly deposit patterned conducting polymer thin films and luminescent polymers for the fabrication of polymer LEDs [J. Bharathan, Y. Yang, Appl. Phys. Lett., 1998, 72, 2660]. However, the thin film devices fabricated by these printing processes have sacrificed performance compared with their counterparts fabricated by other techniques, largely due to the ink materials used not being formulated or designed for printing applications. It will be appreciated that there is a need in the art for the preparation and formulation of luminescent inks designed for the direct printing of optoelectronic devices.
Traditionally, ink is used for writing, painting, and coloring on paper or textile. Commercial inks are designed for these purposes with various colors and properties. When using an ink-jet printer, the ink composition should possess properties that allow it to be jetted appropriately from the printer. During ink jet printing, droplets of ink are expelled from tiny nozzles onto a recording medium. The ink should form a meniscus at each nozzle where it is expelled in the form of droplet. After a droplet is expelled, additional ink surges into the nozzle to reform the meniscus. The ink""s color, viscosity, and surface tension are important to successful printing. Many commercial inks are suitable for general printing applications, but not for thin film printing used to fabricate advanced optoelectronic devices.
The present invention is directed to luminescent ink for printing of organic luminescent devices. The luminescent ink (L-ink) includes a luminescent organic compound, an inert solvent, and a functional additive in an amount sufficient to modify the viscosity of the L-ink to a viscosity in the range up to 90,000 cp, and in an amount to modulate optical and charge transporting properties. When used with stamping and screen printing applications, the L-ink can have a very high viscosity, up to 90,000 cp, preferably up to 70,000 cp, and more preferably between 500 and 10,000 cp. But when used with ink-jet printing, the L-ink viscosity is preferably in the range from about 3 to 100 cp, and more preferably from about 3 to 15 cp.
Functional additives are molecules which are included in the L-ink composition to adjust its viscosity, but which do not adversely affect the electronic properties of the luminescent organic compound and may provide useful electronic properties. The addition of low molecular weight functional additives can be used to lower the viscosity of the L-ink, particularly L-inks containing viscous luminescent polymers. In contrast, high molecular weight functional additives can effectively increase the L-ink viscosity, especially when the L-ink contains low viscosity luminescent small molecules. Functional additives can include small molecules that possess luminescent properties by themselves. Functional additives can include small molecules that possess electron-transporting properties or hole-transporting properties. Functional additives can include a surface energy controlling substance or surfactant to enhance the morphology of the printed ink, to improve the ink/substrate-contacting angle, and to assist in electron and hole transport.
When the L-ink viscosity needs to be lowered, the functional additives are preferably small molecules having a molecular weight less than 5000 g/mol, and more preferably less than 800 g/mol. Typical low-weight organic luminescent molecules that can be used include anthracene, tris(8-hydroxy quinoline) aluminum (Alq3), 4,4xe2x80x2-N,Nxe2x80x2-dicarbazole-biphenyl, distyrylbenzene, 3-(2-benzothiazolyl)-7-(diethylamino)-coumarin, and rubrene. The concentration of low-weight functional additives preferably ranges between 0.5 to 40 weight percent of the solid ingredients.
When the L-ink viscosity needs to be increased, the functional additives are preferably organic luminescent polymers having a molecular weight larger than 5000 g/mol. Typical high-molecular weight organic luminescent molecules that can be used include poly(vinyl carbazole), polyfluorene, such as poly(9,9xe2x80x2-dialkylfluorene), poly(phenylene) derivatives, such as poly(2-silyl-phenylenevinylene) and poly(2,5-dialkoxy-phenylenevinylene), poly(3-alkyl thiophene), and poly(aromatic oxadiazole). The concentration of high-weight functional additives preferably ranges between 0.5 to 40 weight percent of the solid ingredients.
Typical electron-transporting molecules include, but are not limited to, organic compounds containing aromatic oxadiazoles, triazoles, and quinolines, or their combination. Typical hole-transporting molecules include, but are not limited to, from organic compounds containing aromatic amines, carbazoles, thiphenes, copper phthalocyanine (CuPc), poly(N-vinyl-carbazole), and polythiophene derivatives.
Various surfactants can be used as functional additives. Currently preferred surfactants include those with an ionic (anionic or cationic) or neutral terminal and alkyl terminal, having the following general formula:
MYO3R (anionic)
Where R is CnH2n+l, or OCnH2n+1 with n ranging from 4 to 40; and Y is S or P; and
MXR3Rxe2x80x2(cationic)
Where M is Na+, K+, Li+, Mg+2, or H+; X is Clxe2x88x92, Brxe2x88x92, Ixe2x88x92, SO4xe2x88x922; R is methyl or ethyl, Rxe2x80x2 is alkyl Cn where n is 6-40; and
(CH2CH2O)n(CH2CHCH3O)m (neutral)
Where n and m range from 0 to 100 and n+m ranges from 6 to 200.
The functional additives can be used to modify the charge transporting ability of the L-ink to obtain a balanced mode. As used herein, a balanced mode means the barrier height from the cathode metal (work function) to the printed luminescent film (LUMO level) is substantially equal to the barrier height from anode ITO (work function) to the luminescent film (HOMO level). A balanced mode also means that the charge transporting mobility of electrons (injected from the cathode) is substantially equal to the holes (injected from anode).
Cross-linking agents can be included in the L-ink to permit cross-linking the printed L-ink. Cross-linking can give the thin film better thermal stability and facilitate fabrication of multi-layer luminescent devices. Cross-linking agents are preferably added to the L-ink in a concentration in the range from about 0.5 to 30 wt. % of the solid ingredients, and more preferably from about 1 to 10 wt. % of the solid ingredients.
The L-ink solvent preferably has a boiling point in the range from about 40xc2x0 C. to about 200xc2x0 C. Some currently preferred solvents include, but are not limited to, water, alcohol, dioxane, toluene, chloroform, tetrahydrofuran, dichlorobenzene, 1,2-dichloroethane, and xylene. The L-ink preferably contains from about 50% to about 99.8% solvent, by weight.
The present invention also includes a method of fabricating organic luminescent devices. Such luminescent devices will include a first electrode. The L-ink is printed onto the first electrode surface to form a thin film of luminescent material. The L-ink is preferably printed by ink-jet printing (including bubblejet printing), stamp printing, or screen printing. A second electrode is electrically coupled to the thin film of luminescent material. Additional thin films of luminescent material can be printed over the first thin film as needed to fabricate the device. Such thin films can be printed with different luminescent inks. A variety of devices can be fabricated using L-inks within the scope of the present invention, such as LED, thin film transistor, photovoltaic solar cell, electrochemical luminescent display device, electrochromic display device, and electroluminescent flat-panel display device.